Molecular adsorption-induced doping and scattering play a central role in the detection mechanism of graphene gas sensors. However, while the doping contributions in electric field-enhanced gas sensing is well studied, an understanding of the effects of scattering is still lacking. In this work, the scattering contribution of the graphene−molecule van der Waals (vdW) complex is studied under various electric fields and the associated vdW bonding retention in the complex is investigated. We show that contrary to the generally opined view, doping does not always dominate the graphene−molecule vdW complex interaction and consequently the conductivity response in graphene sensors, rather the vdW complex interaction only shows doping-dominated interaction at zero electric fields while scattering increases with electric field modulation. The experimentally observed electric fielddependent scattering response agrees with electron difference density analysis from density functional theory (DFT) calculations, which shows that scattering is directly dependent on the electric field-induced molecular reorientation as well as the redistribution and delocalization of charge in the graphene−gas molecule vdW complex. Furthermore, "vdW bonding memory", i.e., retention of electric field-induced vdW bonding states after turning off the electric field, is observed and shown to result from the high binding energies of the vdW complexes, which are an order of magnitude higher than the sensing measurement thermal energy. This vdW bonding memory in the graphene−molecule complexes is important for the molecular identification of adsorbed gases based on their tunable charge transfer characteristics.
Graphene’s inherent nonselectivity
and strong atmospheric
doping render most graphene-based sensors unsuitable for atmospheric
applications in environmental monitoring of pollutants and breath
detection of biomarkers for noninvasive medical diagnosis. Hence,
demonstrations of graphene’s gas sensitivity are often in inert
environments such as nitrogen, consequently of little practical relevance.
Herein, target gas sensing at the graphene–activated carbon
interface of a graphene-nanopored activated carbon molecular-sieve
sensor obtained via the postlithographic pyrolysis of Novolac resin
residues on graphene nanoribbons is shown to simultaneously induce
ammonia selectivity and atmospheric passivation of graphene. Consequently,
500 parts per trillion (ppt) ammonia sensitivity in atmospheric air
is achieved with a response time of ∼3 s. The similar graphene
and a-C workfunctions ensure that the ambipolar and gas-adsorption-induced
charge transfer characteristics of pristine graphene are retained.
Harnessing the van der Waals bonding memory and electrically tunable
charge-transfer characteristics of the adsorbed molecules on the graphene
channel, a molecular identification technique (charge neutrality point
disparity) is developed and demonstrated to be suitable even at parts
per billion (ppb) gas concentrations. The selectivity and atmospheric
passivation induced by the graphene–activated carbon interface
enable atmospheric applications of graphene sensors in environmental
monitoring and noninvasive medical diagnosis.
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